(a) Field of the Invention
The present invention relates to a heat sensitive transfer method, in particular a heat sensitive transfer method which is capable of obtaining a high density transfer image with microenergy by the use of a transfer sheet having a leuco dye-containing transfer layer and a receiving sheet having a developercontaining receiving layer, and further is capable of obtaining transfer images of uniform image density even when a number of transfer operations are repeated using one and the same transfer sheet.
(b) Description of the Prior Art
As heat sensitive transfer mediums there have been usually known the following ones:
(i) one comprising the combination of a transfer sheet prepared by applying a thermo-sublimating dye on a substrate with a receiving sheet for receiving a thermo-sublimating dye image by virtue of thermal printing from the substrate surface side of said transfer sheet, PA1 (ii) the one comprising the combination of a transfer sheet prepared by applying a thermo-fusible substance and a colorant (pigment or dye) on a substrate with a receiving sheet, and the like.
However, the heat sensitive transfer medium referred to in the preceding (i) was defective in that as a heat-subliming dye is used therein, the dye image formed on the receiving sheet is inferior in preservability and therefore there is caused the necessity of overcoating the transfer image.
The heat sensitive transfer referred to in the preceding (ii) was defective in that as the transfer layer is formed by dispersing the pigment or dye in the thermo-fusible substance, when incorporating a large amount of pigment or dye therein for the purpose of obtaining a high density image, its transfer coefficiency is rather deteriorated and consequently it is difficult to obtain a high density image, and further when using a large amount of thermo-fusible substance for the purpose of increasing its heat sensitivity, as a large amount of thermo-fusible substance transfers toward the receiving sheet side, it becomes difficult to strip the transfer sheet and the receiving sheet smoothly and consequently the fine-lined image area becomes blurred, and the like.
In addition, the heat sensitive transfer medium (iii) is known which comprises carrying each of the substances, which thermally react with each other for color-forming, on separate substrates and making these carrying layers face-contact each other to thereby effect thermal printing.
However, this heat sensitive transfer medium (iii) is defective in that as this medium is a reactive type, mere transfer of the transfer layer to the receiving layer at the time of face-contacting can not achieve a sufficient color-forming reaction and consequently a low density image is produced, and that when thermal printing is effected under the heating conditions of high temperature and long time, a higher density image is produced on the receiving sheet but on the other hand an undesirable color-forming reaction is caused (namely, an image is formed) on the transfer sheet.
The inventors of this application have previously proposed various heat sensitive transfer mediums which are capable of improving the above mentioned drawbacks. The heat sensitive transfer mediums of this type can obtain a multiplicity of copies by effecting heat transfer continually in the manner of using one and the same transfer sheet and exchanging the receiving sheets one after another. This heat sensitive transfer can be achieved in the manner of using a heat sensitive type printer equipped with a thermal head and transferring the transfer sheet against this thermal head. In this instance, however, there are caused problems that smudge on the non-image area (scumming) is caused on the copy surface, the obtained image is blurred and the like depending on the kinds of the transfer sheet and the receiving sheet to be used.